Panels used as tiles or walls fall into the category of building products and provide architectural value, acoustical absorbency, acoustical attenuation and utility functions to building interiors. Commonly, panels, such as acoustical panels, are used in areas that require noise control. Examples of these areas are office buildings, department stores, hospitals, hotels, auditoriums, airports, restaurants, libraries, classrooms, theaters, and cinemas, as well as residential buildings.
To provide architectural value and utility functions, an acoustical panel, for example, is substantially flat and self-supporting for suspension in a typical ceiling grid system or similar structure. Thus, acoustical panels possess a certain level of hardness and rigidity, which is often measured by its modulus of rupture (“MOR”). To obtain desired acoustical characteristics, an acoustical panel also possesses sound absorption, as well as transmission reduction properties.
Sound absorption is typically measured by its Noise Reduction Coefficient (“NRC”) as described in ASTM C423. NRC is represented by a number between 0 and 1.00, which indicates the fraction of sound reaching it that is absorbed. An acoustical panel with an NRC value of 0.60 absorbs 60% of the sound that strikes it and deflects 40% of the sound. Another test method is estimated NRC (“eNRC”), which uses an impedance tube as described in ASTM C384.
The ability to reduce sound transmission is measured by the values of Ceiling Attenuation Class (“CAC”) as described in ASTM E1414. CAC value is measured in decibels (“dB”), and represents the amount of sound reduction when sound is transmitted through the material. For example, an acoustical panel with a CAC of 40 reduces transmitted sound by 40 decibels. Similarly, sound transmission reduction can also be measured by its Sound Transmission Class (“STC”) as described in ASTM E413 and E90. For example, a panel with an STC value of 40 reduces transmitted sound by 40 decibels.
Acoustical panels made in accordance with various industry standards and building codes have a Class A fire rating. According to ASTM E84, a flame spread index less than 25 and a smoke development index less than 50 are required. Airflow resistivity, a measurement of the porosity of a mat, is tested according to modified ASTM C423 and C386 standards. In addition, MOR, hardness and sag of acoustical panels are tested according to ASTM C367. Increased porosity of a base mat improves acoustical absorbency, but it is not measured by any specific industry standard or building code.
Currently, most acoustical panels or tiles are made using a water-felting process preferred in the art due to its speed and efficiency. In a water-felting process, the base mat is formed utilizing a method similar to papermaking. One version of this process is described in U.S. Pat. No. 5,911,818 issued to Baig, herein incorporated by reference. Initially, an aqueous slurry including a dilute aqueous dispersion of mineral wool and a lightweight aggregate, is delivered onto a moving foraminous wire of a Fourdrinier-type mat forming machine. Water is drained by gravity from the slurry and then optionally further dewatered by means of vacuum suction and/or by pressing. Next, the dewatered base mat, which may still hold some water, is dried in a heated oven or kiln to remove the residual moisture. Panels of acceptable size, appearance and acoustic properties are obtained by finishing the dried base mat. Finishing includes surface grinding, cutting, perforation/fissuring, roll/spray coating, edge cutting and/or laminating the panel onto a scrim or screen.
A typical acoustical panel base mat composition includes inorganic fibers, cellulosic fibers, binders and fillers. As is known in the industry, inorganic fibers can be either mineral wool (which is interchangeable with slag wool, rock wool and stone wool) or fiberglass. Mineral wool is formed by first melting slag or rock wool at 1300° C. (2372° F.) to 1650° C. (3002° F.). The molten mineral is then spun into wool in a fiberizing spinner via a continuous air stream. Inorganic fibers are stiff, giving the base mat bulk and porosity. Conversely, cellulosic fibers act as structural elements, providing both wet and dry base mat strength. The strength is due to the formation of countless hydrogen bonds with various ingredients in the base mat, which is a result of the hydrophilic nature of the cellulosic fibers.
A typical base mat binder used is starch. Typical starches used in acoustical panels are unmodified, uncooked starch granules that are dispersed in an aqueous panel slurry and distributed generally uniformly in the base mat. Once heated, the starch granules become cooked and dissolve, providing binding ability to the panel ingredients. Starches not only assist in the flexural strength of the acoustical panels, but also for hardness and rigidity of the panel. In certain panel compositions having a high concentration of inorganic fibers, a latex binder is used as the primary binding agent.
Typical base mat fillers include both heavyweight and lightweight inorganic materials. A primary function of the filler is to provide flexural strength and contribute to hardness of the panel. Even though the term “filler” is used throughout this disclosure, it is to be understood that each filler has unique properties and/or characteristics that can influence the rigidity, hardness, sag, sound absorption and reduction in the sound transmission in panels. Examples of heavyweight fillers include calcium carbonate, clay or gypsum. An example of a lightweight filler includes expanded perlite. As a filler, expanded perlite has the advantage of being bulky, thereby reducing the amount of filler required in the base mat.
One disadvantage of expanded perlite is that the perlite particles tend to fill the pores in the base mat and seal its surface, which compromises the sound absorption capacity of the panel. Furthermore, expanded perlite is relatively fragile and frangible during the manufacturing process. In general, the greater the amount of expanded perlite used, the poorer the panel acoustic absorption properties. The expansion of perlite consumes a significant amount of energy. Expanded perlite is formed when perlite ore is introduced into an expanding tower that is heated to about 950° C. (1750° F.). Water in the perlite structure turns to steam and the resulting expansion causes the perlite to “pop” like popcorn to reduce the density to about one-tenth of the unexpanded material. The lower bulk density of expanded perlite enables it to flow upward in the expanding tower and be collected by a filtering device. This process uses a relatively large amount of energy to heat all of the perlite to a temperature sufficient to vaporize the water within it.
Given the current trends in the building industry, there is a desire for products which are environmentally friendly, i.e., made with processes that result in reduced global warming, acidification, smog, eutrophication of water, solid waste, primary energy consumption and/or water effluent discharge. Naturally growing, renewable materials can be used to produce environmentally friendly building products. In the building industry, a widely used renewable material is lumber, but it provides little acoustical absorption. There is a large amount of agricultural waste and byproducts, as well as lumber and furniture industry waste that is readily available but has limited use in building materials production.
Cellulosic fibers are made by pulping ligno-cellulosic materials such as wood, straw, bamboo and others to break the plant material into its individual fiber cells either chemically or mechanically. A common chemical pulping method uses sodium sulfide, sodium hydroxide or sodium sulfite to dissolve the lignin at about 150° C. (302° F.) to about 180° C. (356° F.), reducing the fiber's biomass by about 40-60%. A thermal-mechanical pulping method subjects wood chips to high temperatures (about 130° C. (266° F.)) and high pressure (about 3-4 atmospheres (304-405 kPa)), causing the lignin to soften and allowing fiber cells to be mechanically torn apart. Disruption of the lignin bond causes the defiberization of the raw material with a resulting loss in its biomass of about 5-10%. Both chemical and thermal-mechanical pulping processes require significant amounts of energy to reduce the ligno-cellulosic material to its individual fibers. Further, the loss of such a large fraction of the biomass increases the cost of raw materials.
Several United States patents teach using renewable materials in building materials. U.S. Pat. No. 6,322,731 discloses a method for forming a structural panel of indefinite length, comprising an organic particulate base material consisting predominately of rice hulls and a binder. Due to the requirements for structural integrity, the process requires a combination of high temperature and high pressure to form a panel of sufficient strength. The resultant panel has relatively low sound absorption value, due to its high density and low porosity. The thermal and acoustic insulation characteristics are achieved through the encased cavities.
U.S. Pat. No. 5,851,281 discloses a process for manufacturing a cement-waste material composite where the waste material is rice husks. The rice husks are heated to approximately 600° C. (1112° F.) in the absence of oxygen to produce micro-granules.
U.S. Pat. No. 6,443,258 discloses an acoustically absorbent porous panel formed from a cured, aqueous, foamed, cementitious material. The panel provides good acoustical performance with enhanced durability and moisture resistance. Rice hull ash is added to enhance the overall hardness of the foamed cement panel.